This invention relates to a novel wire filter element with mechanically controlled filter slits of precise minute openings in the range of 0.5 to 100 microns, and to a method of manufacturing the novel wire filter element.
Filter elements consisting of trapezoidal shaped wire elements are used in various processes to separate fibrous and coarse material from finer solids and liquids. Typical of such uses are paper pulp separation, water well screens, water treatments, and others. There are other processes which use finely divided catalysts to promote reactions in vessels typically operated at elevated temperatures and pressure where it is economically desirable to use the catalysts for prolonged periods without shutting down the process or without losing or replacing the catalyst. Some of the existing processes retain the catalyst in the reactor, or control the loss to a small amount each day by use of cyclones, filters, electrostatic precipitators, or scrubbers. At times these methods are satisfactory and at other times not. In certain processes the catalyst particle diameters used typically fall in the range of 0.5 to 100 microns. There has been a rapidly increasing use of Slurry Bubble Column Reactors using the very fine sized catalysts in which catalyst retention has been an unsolved problem. This was the conclusion of B. Bhatt et al in their paper entitled "Liquid-Phase Fischer-Tropsch Synthesis in a Bubble Column," published by the U.S. Department of Energy Pittsburgh Technology Center in the compilation "Liquefactions Proceedings", Sep. 22-24, 1992. As stated in the paper: "No single proven technology exists in the public domain" to retain the catalyst.
In tests and other experience with fine particle retention, the tendency of porous metal, fine-wire matts and other designs to rapidly and often permanently plug has been often demonstrated. The shaped-wire screens or filters have a greatly reduced tendency for plugging. However, they have not yet been made with openings fine enough to retain catalysts of the size typically used in Slurry Bubble Column Reactors, or similar applications.
Various attempts have been unsuccessfully made to limit filter slit widths to a precise minute dimension of about 20 microns. Korchi Arai in his U.S. Pat. No. 5,047,148 for "Retained Wire Filter Element" represents the forming of filtering slits with an accuracy of one micron to one millimeter by anchoring a specially shaped wire into matching grooves in a plate. The distance between the grooves in the plate, minus the width of the top surface of the specially shaped wire would determine the filtering slit width with the accuracy claimed. Since the filtering slit width in the Arai technique is subject to the sum of at least two machining tolerances, it appears unlikely that a slit width accuracy represented by Arai can be attained.
Fritted metal and fine-wire screens have been made with openings averaging as small as a few microns. However, it has been found that such filters display wide variances in the size of the space inside the relatively deep structure of the filter medium. The result is that fine particles infiltrate and are trapped inside the structure resulting in plugging and blinding.
This invention overcomes the limitations in heretofore known filter elements having filtering slits of precise minute widths by providing a filtering element and method of manufacturing the element with controlled openings or filtering slits with widths in the range of 0.5 to 100 microns. Thus, this invention will permit the retention of catalysts or solids of fine sizes and at the same time achieve the filtering advantages of shaped wire filtering elements. Such filters and their use would enable long operating periods such as a year or more, with catalyst losses confined largely to the fines generated by attrition. The first application of filters embodying my invention in a given process and set of circumstances could use double filters with blowback provision for insurance against temporary plugging from process upsets. Other uses of the filter element of my present invention would include: cross-flow filtration; fluidized bed reactions; other gas-liquid-solids reactions; solid catalyzed liquid-liquid reactions; gas sparging or distribution in beds with fine solids; and instrument connections into beds with fine solids.